In the state of the art, measuring devices are known, which include so-called oscillatory forks. These oscillatory forks are caused to oscillate, and the oscillations, which depend on whether there is contact with the medium and then on characteristics of the medium, are received and evaluated. The oscillations, i.e. their variables, such as frequency or amplitude, depend on fill level, i.e. the degree of covering of the oscillatory fork by the medium, as well as also on density or viscosity of the medium. The dual dependence of the oscillations on density and viscosity of the medium makes the monitoring of density difficult.
Offenlegungsschrift DE 100 57 974 A1 describes such an oscillatory fork and concerns, especially, suppression of the dependence of the oscillations on the viscosity of the medium. The effects of viscosity can be reduced, according to this Offenlegungsschrift, by having a phase other than 90° between exciter signal and received signal. For instance, in the case of liquid media, a desired phase is 70°. Such a phase largely eliminates the effect of viscosity.
In order to find a phase difference, at which viscosity changes have no effects on the frequency of the oscillations, for example, curves of different media are recorded showing phase difference between transmitted and received signals as a function of frequency of the transmitted signal. The intersection of the curves yields the sought phase difference. This is described, for example, in EP 0 985 916 A1 of the present assignee.